Manslaughter is an unlawful killing that doesn't involve malice aforethought—intent to seriously harm or kill, or extreme, reckless disregard for life. The absence of malice aforethought means that manslaughter involves less moral blame than either first or second degree murder. (But plenty argue that some instances of felony murder, a form of first degree murder, involve less blameworthiness than some instances of manslaughter.) Thus, while manslaughter is a serious crime, the punishment for it is generally less than that for murder.
The two main variations of manslaughter are usually referred to as voluntary and involuntary manslaughter.
This is often called a "heat of passion" crime. Voluntary manslaughter occurs when a person:
For "heat of passion" to exist, the person must not have had sufficient time to "cool off" from the provocation. That the killing isn't considered first or second degree murder is a concession to human weakness. Killers who act in the heat of passion may kill intentionally, but the emotional context is a mitigating factor that reduces their moral blameworthiness.
The classic example of voluntary manslaughter involves a husband who comes home unexpectedly to find his wife committing adultery. If the sight of the affair provokes the husband into such a heat of passion that he kills the paramour right then and there, a judge or jury might very well consider the killing to be voluntary manslaughter.
Involuntary manslaughter often refers to unintentional homicide from criminally negligent or reckless conduct. It can also refer to an unintentional killing through commission of a crime other than a felony.
The subtleties between murder and manslaughter reach their peak with involuntary manslaughter, particularly because an accidental killing through extreme recklessness can constitute second degree murder.
Facts: Fast Boyle is walking along a busy street. Clay bumps into Boyle and continues walking without saying, "Sorry." Angered by Clay's rudeness, Boyle immediately pulls out a gun and kills Clay.
Verdict: Boyle could probably be convicted of second degree murder, because he killed Clay intentionally. A judge or jury is unlikely to conclude that the killing was premeditated, which would have elevated the shooting to first degree murder. On the other hand, this wasn't the kind of heat-of-passion killing that equals voluntary manslaughter. While Boyle might have been provoked in some sense, the circumstances weren't so extreme to cause a reasonable person to lose control.
Facts: Standing next to each other in a bookstore a few feet away from the top of a flight of stairs, Marks and Spencer argue over the proper interpretation of free will in Hobbes's philosophy. The argument becomes increasingly animated and culminates when Spencer points a finger at Marks and Marks pushes Spencer backwards. The push is hard enough to cause Spencer to fall backwards and down the stairs. Spencer dies from the resulting injuries.
Verdict: Marks would probably be guilty of involuntary manslaughter. It was criminally negligent of him to shove a person standing near the top of a stairway. But the circumstances don't seem to suggest that his behavior was so reckless as to demonstrate extreme indifference to human life, which would have elevated the crime to second degree murder. If the evidence had indicated that Marks intended to kill Spencer with the push, a judge or jury would have had to determine whether the extent of the provocation made the homicide voluntary manslaughter.
Facts: Lew Manion comes home to find that his wife Lee has been badly beaten and sexually abused. Manion takes Lee to the hospital. On the way, Lee tells Manion that her attacker was Barnett, the owner of a tavern that she and Manion occasionally visit. After driving Lee home from the hospital about four hours later, Manion goes to a gun shop and buys a gun. Manion then goes to the tavern and shoots and kills Barnett.
Verdict: Manion could be convicted of first degree murder, because the time for reflection and his purchase of the gun indicates premeditation and deliberation. Voluntary manslaughter is a somewhat less likely alternative because a judge or jury could find that the heat of passion had cooled, even though Manion remained angry at the time he acted.
Manslaughter convictions often result in prison time. As an example, in a 2004 New York decision, an appeals court upheld the following sentence in a driving-while-intoxicated case where there was a collision and the defendant's passenger died as a result: three to nine years for second-degree manslaughter, concurrent with two to six years for second-degree vehicular manslaughter. (People v. Yanus, 13 A.D.3d 804 (2004).)
Keep in mind, of course, that the sentence in any any case depends not only on the jurisdiction's laws, but also on the court's evaluation of the circumstances and the defendant.